1. Field of the Invention
This invention relates to a cylinder locking device for locking and keeping stationary the piston rod of a hydraulic cylinder at a desired position.
2. Description of the Prior Art
Conventional cylinder locking devices of the foregoing type are typically constructed such that a brake member having an eccentric hole is fitted rotatably on a piston rod and when locking the piston rod, the brake member is rotated about an axis eccentric to the axis of the piston rod. However, since the piston rod is locked by the eccentric rotation of the brake member, the piston rod deforms as being twisted; consequently, shearing force is generated in the piston rod, or a bearing portion of the piston rod is deformed or worn.
In view of the above, the present inventor has proposed in U.S. Pat. No. 4,791,855 a cylinder locking device which is capable of locking the piston rod of a hydraulic cylinder at a desired position without accompanying deformation of the piston rod and wear of bearings. This cylinder locking device is constructed such that two semicylindrical brake members are formed by dividing a thick cylinder having eccentric outer and inner circumferential portions axially into two halves at its thickest and thinnest portions, the two brake members are disposed rotatably in a housing, the piston rod is disposed inside the two brake members, and when locking the piston rod, the two semicylindrical brake members are rotated in opposite directions.
The foregoing locking device including the two semicylindrical brake members can attain locking without giving shearing force or the like to the piston rod. However, a coiled spring for urging the two semicylindrical brake members in their locking directions and an actuator for urging them in their unlocking directions become large in size; thus, the locking device as a whole becomes large in size and becomes complicated in structure.